This invention relates to apparatus for producing an output signal varying in accordance with a Doppler frequency and more particularly to Doppler ultrasonic flowmeters compensated for changes in the frequency of the transmitted signal.
In ultrasonic Doppler flowmeters an oscillator having a frequency f.sub.o is connected to apply this frequency to an ultrasonic transducer so that an ultrasonic beam is propagated in a flowing fluid at an angle .theta. with respect to the direction of the flow. Part of the ultrasonic energy is reflected back to the transducer by air bubbles or particles in the fluid. Because the reflectors are traveling at the same velocity as the fluid, the frequency of the reflected energy is shifted from the transmitted frequency f.sub.o by an amount F.sub.d by virtue of the Doppler effect. For quantity F.sub.d is given by the well known equation EQU F.sub.d =2V.sub.f f.sub.o (Cos.theta./C) (1)
where V.sub.f is the fluid velocity and C is the acoustic velocity in the fluid.
The usual ultrasonic Doppler flowmeter uses a monostable multivibrator triggered by the signal having a frequency F.sub.d. Such a multivibrator generates a pulse of relatively fixed amplitude and duration with a repetition frequency F.sub.d. A lowpass filter is then used to extract the average or dc level of the pulse train. It will be understood that any drift in the monostable multivibrator that would tend to change the pulse duration will introduce an error into the average level of the pulse train that is not related to the Doppler frequency. Additionally, reference to equation (1) will show that the Doppler frequency itself will not represent the fluid velocity if the transmitted frequency f.sub.o should change due to change in the frequency of the oscillator.
In order to overcome these sources of error, a synchronous frequency-to-voltage converter is proposed in which the pulse width of the pulse output from the converter is made to vary inversely with the frequency of the oscillator. In this manner it is possible to compensate the ultrasonic Doppler flowmeter for changes in oscillator frequency and for changes created by drift of the multivibrator that is normally used in the frequency-to-voltage converter.
An object of this invention is to remove errors in ultrasonic Doppler flowmeters due to variations in the pulse width and the oscillator frequency by making the pulse width inversely proportional to the oscillator frequency.